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Press Release

11 October 1995

The Royal Swedish Academy of Sciences has decided to
award the 1995 Nobel Prize in Physics for pioneering
experimental contributions to lepton physics with one half to
Martin L. Perl, Stanford University, Stanford, California,
USA for the discovery of the tau lepton
and with one half to Frederick Reines, University of
California, Irvine, California, USA for the detection of the
neutrino.

Discoveries of two of nature's sub-atomic particles
rewarded

Mankind seeks his place in nature. He
endeavours to find answers to philosophical and physical
questions alike. The home of mankind, the Universe, was created
in a Big Bang. "What does this Universe consist of?" - "What are
the smallest constituents of the Universe and what are their
properties?" - "What can they tell us of the history of the
Universe and of its future?" etc. This year's laureates have in
this search made lasting contributions: They have discovered two
of nature's most remarkable subatomic particles.

Martin L. Perl and his colleagues discovered, through a
series of experiments between 1974 and 1977, at the Stanford Linear
Accelerator Center (SLAC) in the USA, that the electron has a
relative some 3 500 times heavier, which is called the tau.

Frederick Reines made pioneering contributions during the
1950s together with the late Clyde L. Cowan, Jr., which led to
their being able to demonstrate experimentally the existence of
the antineutrino of the electron.

Martin Perl's discovery of the tau was the first
sign that a third "family" of fundamental building blocks
existed. Some years later a further building block was discovered
- one of the family's two quarks, the bottom quark. Not until 18
years later was its other quark, the top quark, discovered. The
existence of the third family is very important for physicists'
confidence in the present theoretical model for understanding the
properties of nature's smallest constituents. This is called the
standard model. Without a third family, the model would have been
incomplete and unable to admit what is termed the Charge and
Parity (CP) violation, a violation of a fundamental principle of
symmetry which, among other things, regulates particle decay
(Nobel Prize to Cronin and Fitch
1980). If a fourth family of quarks and leptons is
discovered, this may mean that the standard model must be revised
and more extensive reconstruction within elementary-particle
physics commenced.

Frederick Reines' and Clyde L. Cowan's first observation
of neutrinos was a pioneering contribution that opened the
doors to the region of "impossible" neutrinoexperiments. Nowadays
we are attempting to capture neutrinos in cosmic radiation that
may originate in the sun or in supernovas (exploding stars).
Because of the reluctance of neutrinos to react with atomic
nuclei and thus allow themselves to be captured, very large
detector volumes are required for these experiments. While Reines
and Cowan in the 1950s managed with about half a cubic metre of
water in their detector, large-scale experiments in the 1990s use
many thousand cubic metres. Some experiments have even used
surrounding sea or ice as their detector volume.

Nature's building blocks and their
family structure
The smallest of nature's structures to have been studied so far
are twelve types of matter particles - six quarks and six
leptons. They each have their anti-particle, a sort of "mirror
image" of the particle. (The name of a particle also includes its
anti-particle.) As well as these quarks and leptons, there are
other types of subatomic particles called force particles,
since they are responsible for three of our known forces,
strong force, electromagnetic force and weak
force. Gravitational forces operate outside the scope of
these. The most essential difference between quarks and leptons
is that leptons are not affected by the strong force.

A remarkable property of matter particles is that they exhibit
"family affiliation". They come in three families, each
consisting of two quarks and two leptons (Fig. 1). In many ways
the three families behave as copies of one another. "Is there a
fundamental principle to justify the existence of just three
families?" is one of the unanswered questions of physics.

The quarks of the first family are "up quarks" and "'down
quarks". Its lepton members are the electron and the
electron-neutrino. The two quarks build protons and neutrons,
which in turn form atomic nuclei and hence over 99% of all the
earth's matter. The small remainder is electrons. The
electron-neutrino can be imagined very roughly as an electron
deprived of charge and mass. Whether an insignificant amount of
mass nevertheless remains, is another unanswered question. It was
this electron-neutrino that Reines and his colleague Cowan, both
then employed at the Los Alamos Scientific Laboratory, managed to
capture.

The discovery of the tau During the 1960s many research groups
were carrying out experiments one aim of which was to discover
new charged particles, including new leptons. One approach was to
search for the new particles in the products of decay of the
particles that were available, e.g. kaons. Another way was to
attempt to produce them in an accelerator, e.g. in collisions
between high-energy electrons and a target. Martin Perl
was a member of a team performing such an experiment at SLAC in
1966, but no new charged leptons were found. In 1973 a new
machine started operation at SLAC - an electron-positron collider
called SPEAR. A collider such as this is a lepton-hunter's dream
since the mechanism for possible production of new charged
leptons (X+, X-) is simple and easy to
interpret:

electron + positron -> X+ +
X-

The SPEAR collider offered Perl an
exceptional opportunity to continue his earlier hunt for new
leptons, this time in a new and earlier inaccessible energy
region of about 5 GeV (5 thousand million electron volts). After
only a year came the first hint that something exciting was in
the offing - something that could be signalling the production of
a new type of lepton. The next year Perl and his co-workers
published the first results. But a few more years were to pass
before they could be certain that they had in fact discovered a
lepton. The new lepton was designated with the Greek letter tau,
standing for the first letter of the word triton, third.

Fig 1. The elementary particles of the
standard model - a new periodic system. The figure represents
a Triton.

Perl's and co-workers'
experiment
The experiment recorded frontal collisions between electrons and
their antiparticles, positrons. A large cylindrical detector
placed in a magnetic field surrounded the collision area. The
detector consisted of many components including a number of wire
spark chambers together with shower counters constructed of lead
scintillators and a few proportional chambers. The first
indication of a possible new phenomenon was that the research
team observed 24 events of the type

electron + positron -> electron + antimuon + i.p.

or

electron + positron -> positron + muon + i.p.

where i.p. stands for invisible particles;
those that left no trace in the detector. Thus only one electron
(or positron) and an antimuon (or muon) with the opposite sign on
its charges were detected. Applying the law of conservation of
energy, Perl and his co-workers found that they had produced at
least two invisible particles.

One possible interpretation of these events was that a pair of
heavy leptons, later termed tau particles, had been produced
first:

electron + positron -> tau + antitau

But these were expected to decay very
rapidly and the observed electrons and muons were therefore
interpreted as products of decay from reactions:

tau -> electron (or muon) + neutrinos

antitau -> antimuon (or positron) + neutrinos

The invisible particles were neutrinos,
which with their notorious lack of sensitivity to their
surroundings disappeared without visible trace (Fig. 2). However
they made themselves felt when the energy balance was to be
accounted for. They had taken with them a respectable proportion
of the energy (cf. below).

Perl's and co-workers' hypothesis was tested in a new series of
observations over many years. It gradually became clear that the
tau had passed the test and thereby met all the possible
requirements of a heavier relative to the electron and the muon.
Like these, the tau also has its very own neutrino - the tau
neutrino.

Fig. 2. Interpretation of a typical
electron-muon trace from the SPEAR detector. The two heavy
leptons decay within millimetres of the point of collision
and cannot be seen directly. The neutrinos are also
invisible. Only the charged particles e and µ are
detected.

Energy conservation law cue for
neutrino's entrance
The neutrino hypothesis is some 40 years older. The neutrino "was
born" as a hypothetical particle in a letter written in 1930 by
Wolfgang Pauli
(Nobel Prize 1945). At that time it was known that many atomic
nuclei ended their lives by emitting an electron. This process,
termed beta decay, caused researchers many headaches, among
others that one of the sacred laws of physics - the law of
conservation of energy - appeared not to apply. To restore order
in the statute book of physics Pauli offered what he called a
"desperate solution" - the nucleus did not emit the electron
alone. It was accompanied by another subatomic particle which
lacked electrical charge and reacted very little with its
environment. The small particle, which came to be called the
neutrino, took part of the energy with it and disappeared without
trace into nothingness. The energy balance proved to be as
expected as long as account was taken of the proportion the
neutrino had removed.

Pauli thought he had done "a frightful
thing", as he called it, by proposing a particle that could never
be discovered. It took three decades and the ingenuity of Reines
and Cowan to bring the neutrino to light.

The discovery of the neutrino
Pauli's neutrino hypothesis may have been "frightful" but it was
also extremely attractive. It saved the energy conservation law
and simultaneously solved many other riddles. The neutrino
hypothesis was used by Enrico Fermi (Nobel
Prize 1938) in a masterly manner to formulate a theory for one of
the natural forces, the weak force. This splendid theory lent
great credibility to the hypothesis that the neutrino is created
simultaneously with the electron every time a nucleus
disintegrates through beta decay. But how to produce conclusive
proof that the neutrino existed? Researchers Hans Bethe (Nobel
Prize 1967) and Rudolf Peierls had evaluated the probability of
stopping neutrinos produced in the beta decay of radioactive
nuclei and found that it was so minimal that a target several
light years thick would be needed to capture these neutrinos
efficiently. When the first nuclear reactors were built during
the 1940s, Fermi was one of those who realised that the reactors
could serve as intensive neutrino sources. It was estimated that
the reactors would be able to give a neutrino flow of about
1012-1013 per second and cm2.
This was many orders of magnitude greater than what was obtained
from radioactive sources.

In 1953 Reines and Cowan proposed a reactor
experiment to capture neutrinos. The reaction to be studied
was

antineutrino + proton -> neutron + positron.

Despite the great intensity of the
neutrinos the reactor delivered, such a low counting speed was
expected for this reaction that the attempt appeared to be
bordering on the impossible. Reines and Cowan realised the
importance of detecting both the neutron and the positron to
reduce the risk of erroneous interpretation. After a first trial
at the Hanford reactor, Reines and Cowan went to work at the
Savannah River Plant.

The target in the Reines-Cowan experiment
consisted of approximately 400 litres of water containing cadmium
chloride placed between large liquid scintillation detectors. The
course of events for the reaction sought is as follows (cf.
formula above): The neutrino collides with a proton in the water
and creates a positron and a neutron. The positron is slowed down
by the water and destroyed together with an electron (matter
meets antimatter), whereupon two photons (light particles) are
created. These are recorded simultaneously in the two detectors
(Fig. 3). The neutron also loses velocity in the water and is
eventually captured by a cadmium nucleus, whereupon photons are
emitted. These photons reach the detectors a microsecond or so
later than those from the destruction of the positron and give
proof of neutrino capture.

Fig 3. Schematic picture of the
neutrino detector of Reines and Cowan (see explanation in
text).

There were struggles with the low counting
speed and high background. During the experiment a few events
were recorded per hour. Nevertheless Reines and Cowan succeeded
in a feat considered to border on the impossible: They had raised
the neutrino from its status as a figure of the imagination to an
existence as a free particle.

Frederick Reines
Born 1918 Paterson, New Jersey, USA. American citizen. Doctor's
degree in physics 1944, New York University. Reines is a member
of the National Academy of Sciences, USA, and a foreign member of
the Russian Academy of Sciences.